1. Field of the Invention
The present invention relates to a method of manufacturing an MEMS (Micro Electrical Mechanical System) device.
2. Description of the Related Art
A micro machining technology that applies a semiconductor micro-fabrication technique is capable of manufacturing a micro structure of a few 100 μm or so. The application of such a technology to various sensors, an optical switch, RF parts, etc. has been discussed. Since a device based on such a micro machining technology is manufactured using a silicon process, the device can be integrated on a chip together with a signal processing system LSI, and a system having a specific function can be constructed on the chip. Such a device is called “MEMS (Micro Electrical Mechanical System) device”.
A conventional MEMS device has been described in, for example, patent documents 1 (Japanese Unexamined Patent Publication No. Hei 9(1997)-15257) and 2 (Japanese Unexamined Patent Publication No. Hei 7(1995)-225240).
Acceleration sensors formed by laminating first and second silicon substrates on each other have been described in the patent document 1. Each of the acceleration sensors has a square-shaped frame (peripheral portion) having an opening, a weight portion disposed in the opening formed inside the frame, and beam portions that elastically support the weight portion at the peripheral portion.
Acceleration sensors each formed integrally with a silicon substrate (device substrate) by a semiconductor manufacturing process have been described in the patent document 2. The acceleration sensor has a frame (peripheral portion) having an opening, a weight portion disposed in the center of the opening, and two beams that support the weight portion elastically and in cantilever manner on the frame. The two beams support the weight portion at the peripheral portion on the upper surface side of the peripheral portion. The lower surface of the acceleration sensor is fixed to a cover (cap substrate) having a thickness substantially identical to that of the acceleration sensor. The cap substrate is laminated on the device substrate formed with such acceleration sensors, followed by being fixed to an UV sheet with an adhesive, and the respective acceleration sensors are separated by dicing.
An example illustrative of the bonding of the substrates to each other in the semiconductor manufacturing process as described in each of the patent documents 1 and 2 has been described in, for example, patent documents 3 (Japanese Unexamined Patent Publication No. Hei 5(1993)-55534) and 4 (Japanese Unexamined Patent Publication No. Hei 8(1996)-125121).
A laminated semiconductor device in which a first SOI substrate and a second SOI substrate are bonded to each other, has been described in the patent document 3. The first SOI substrate has a first device layer and a contact portion formed on the first device layer. The second SOI substrate has a second device layer and an SOG (Spin On Glass) film formed on the second device layer. A concave portion fitted in the contact portion is formed in the SOG film. In the semiconductor device, the first SOI substrate and the second SOI substrate are joined to each other in a state in which the contact portion is being fitted in the concave portion of the SOG film, thereby to achieve electrical contact between the first device layer and the second device layer.
A semiconductor device in which a first semiconductor substrate and a second semiconductor substrate are joined to each other, has been described in the patent document 4. Each of the first and second semiconductor substrates includes a transistor, electrodes potentially connected to the transistor via an interlayer insulating film, and an insulating film formed in a gap between the adjacent electrodes. Concavo-convex patterns sawtooth as viewed in section are formed in the electrodes and the insulating film of the first semiconductor substrate. Concavo-convex patterns 180° out of phase with the concavo-convex patterns of the first semiconductor substrate are formed in the electrodes and the insulating film of the second semiconductor substrate. In the present semiconductor device, the first and second semiconductor substrates are bonded to each other in a state in which the concavo-convex patterns of the first and second semiconductor substrates are being engaged with one another, whereby electrical contacts between the transistors of the respective semiconductor substrates are made.
Since the openings of the frames extend through the first and second substrates in the acceleration sensors described in the patent document 1, the following problems occur upon fractionization of the acceleration sensors by dicing. Firstly, there is a possibility that the characteristic of each acceleration sensor will be deteriorated due to the intrusion of the residual of silicon into the acceleration sensor upon dicing. Secondly, there may be a case in which when the acceleration sensors are directly adhered to the UV sheet, the adhesive is attached to the weight portions, and when the fractionalized acceleration sensors are picked up, the weight portions are not separated from the UV sheet with ease. In such a case, there is a possibility that the beam portions will break down due to the application of excessive stress to the beam portions that support the weight portions. Thirdly, there is a possibility that the thin beam portions will be broken due to pressure of wafer for sweeping or discharging the residual of silicon upon dicing.
Since the cap substrate is bonded to the device substrate and thereafter dicing is done in the acceleration sensors described in the patent document 2, the acceleration sensors are covered with the cap substrate. Thus, the first through third problems can be resolved. That is, since the acceleration sensors are covered with the cap substrate, it is possible to prevent the residual of silicon from being intruded into the acceleration sensors upon dicing, prevent the adhesive from adhering to the weight portions of the acceleration sensors and prevent damage of the beam portions due to the water pressure.
However, the cap substrate needs a film thickness for maintaining a hand ring strength at the time that it is handled as singular or discrete. The cap substrate normally needs a thickness substantially identical to the silicon substrate. As a result, there is a possibility that the thickness of each acceleration sensor will increase due to the bonding of the cap substrate to the device substrate. When the acceleration sensor is mounted to a small-sized device such as a cellular phone, the space required for assembly in the device is restricted and hence the thickness of an acceleration sensor chip is restricted. A cellular phone, which is now ubiquitous, needs to have a chip thickness of 1.2 mm or less as the post-assembly chip thickness. Thus, it is not possible to ignore an increase in the thickness due to the bonding of the cap substrate to the device substrate.
The laminating of the substrates on each other, which has been described in each of the cited documents 3 and 4, is equivalent to such a configuration as to join the substrates to make contact between adjacent devices of the respective substrates. This is not intended for one that pays attention to the problem that the post-bonding substrate becomes thick due to the thickness of the cap substrate. That is, the structure described in each of the cited documents 3 and 4 is of a structure in which substrates each having substantially the same thickness are bonded to each other. A post-bonding substrate is substantially twice as thick as a discrete substrate.